1. Field of the Invention
This invention relates to a thin film amorphous semiconductor memory device and more particularly to such a device that can be formed upon the surface of a substrate.
2. Description of the Prior Art
Both volatile and non-volatile memories can be formed from an amorphous semiconductor devices which are capable of being switched to and from a low-resistance crystalline state. A particular type of memory switching amorphous semiconductor material is the tellurium based chalcogenide class of materials which have the general formula Ge.sub.A Te.sub.B X.sub.C Y.sub.D. Such amorphous high-resistance materials can be placed between a pair of spaced apart electrodes. By applying a voltage pulse (between the electrodes) of greater amplitude than the threshold voltage of the amorphous material, a low-resistance film is formed between the electrodes. In the non-volatile mode, the application of this pulse for a proper duration causes a conducting crystalline filament to form and then remain between the electrodes after the pulse is removed (a set operation). A reset group of pulses of appropriate current and duration causes the crystalline path to return to the relatively amorphous state (reset operation). In a volatile mode, with the application of a limited current, the material will switch from the high-resistance to low-resistance state but the crystalline filament will not be formed. Particular materials that may be employed are disclosed in the Ovshinsky U.S. Pat. No. 3,271,591; the Dewald, et al, U.S. Pat. No. 3,241,009; the Neale Pat. No. 3,669,543; and the Buckley U.S. Pat. No. 3,886,577.
The above-referenced Dewald, et al, and Ovshinsky patents indicate that such chalcogenides can be employed in two different manners, namely, as either a threshold device or a memory storage element. In the storage element mode, the chalcogenide layer provides as a non-volatile memory. In the threshold mode, the chalcogenide layer serves as a threshold switch.
A particular advantage of such chalcogenide memory cells is that they provide a non-volatile memory even if power is lost. Furthermore, they require less surface area per cell than do semiconductor memory cells that require at least two transistors for a static memory or a transistor and capacitance for a dynamic memory However, they still require an isolating diode which, in the past, has been formed in the semiconductor substrate.
The above-referenced Buckley patent discloses the location of such an amorphous memory device in a memory matrix where the device is formed in series with a P-N junction isolating diode, the latter of which is formed in a semiconductor substrate. However, formation of such device requires epitaxial, diffusion and other processes which would not be needed if the memory matrix could be formed on top of the substrate. Conversely, by the formation of the memory matrix on top of a silicon substrate, other active devices and circuits can be first formed in the substrate under the cell.
It is then an object of the present invention to provide an improved amorphous semiconductor memory device.
It is another object of the present invention to provide an improved amorphous semiconductor memory device which is employed in the memory matrix that can be formed on the top of the surface of a substrate.
It is still a further object of the present invention to provide an amorphous semiconductor memory matrix which can be formed on top of a substrate having other active elements formed therein.